Hidden Processes During Seasonal Isolation of a High-Altitude Watershed

Jessica Z Buser-Young,Laura L Lapham,Frederick S Colwell,Kenneth H Williams,Andrew R Thurber

doi:10.3389/feart.2021.666819

Abstract

Biogeochemical processes capable of altering global carbon systems occur frequently in Earth’s Critical Zone–the area spanning from vegetation canopy to saturated bedrock–yet many of these phenomena are difficult to detect. Observation of these processes is limited by the seasonal inaccessibility of remote ecosystems, such as those in mountainous, snow- and ice-dominated areas. This isolation leads to a distinct gap in biogeochemical knowledge that ultimately affects the accuracy and confidence with which these ecosystems can be computationally modeled for the purpose of projecting change under different climate scenarios. To examine a high-altitude, headwater ecosystem’s role in methanogenesis, sulfate reduction, and groundwater-surface water exchange, water samples were continuously collected from the river and hyporheic zones (HZ) during winter isolation in the East River (ER), CO watershed. Measurements of continuously collected ER surface water revealed up to 50 μM levels of dissolved methane in July through September, while samples from 12 cm deep in the hyporheic zone at the same location showed a spring to early summer peak in methane with a strong biogenic signature (&lt;65 μM, δ13C-CH4, −60.76‰) before declining. Continuously collected δ18O-H2O and δ2H-H2O isotopes from the water column exhibited similar patterns to discrete measurements, while samples 12 cm deep in the hyporheic zone experienced distinct fluctuations in δ18O-H2O, alluding to significant groundwater interactions. Continuously collected microbial communities in the river in the late fall and early winter revealed diverse populations that reflect the taxonomic composition of ecologically similar river systems, including taxa indicative of methane cycling in this system. These measurements captured several biogeochemical components of the high-altitude watershed in response to seasonality, strengthening our understanding of these systems during the winter months.

Highlights

Headwater streams, the primary entity in the hierarchy of watersheds (Lowe and Likens, 2005), are estimated to account for nearly three quarters of stream channel length in U.S watersheds (Leopold et al, 1992)
Geomorphology, and biogeochemistry of watersheds have demonstrated carbon and methane emissions originated as terrestrial organic carbon input which is processed along the aquatic continuum (Vannote et al, 1980; Crawford et al, 2013)
Methane concentrations 4 cm within the hyporheic zones (HZ) remained relatively constant around 2.8 μM, with a slight increase to 10.2 μM during April with an δ13C-CH4 value ranging from −56.4 to −60.9 ± 0.2

Summary

Introduction

The primary entity in the hierarchy of watersheds (Lowe and Likens, 2005), are estimated to account for nearly three quarters of stream channel length in U.S watersheds (Leopold et al, 1992). Headwater basins have significant methane fluxes and unexpected dynamics (Schade et al, 2016; Flury and Ulseth, 2019) These include degassing events in peatland (Billett and Harvey, 2013) and Arctic (Street et al, 2016) headwater streams, lending much uncertainty to predictive modeling efforts (Saunois et al, 2020) and underestimations of methane release in extant models (Wallin et al, 2014). While these studies have identified the interplay between headwater stream dynamics and methane emission, these dynamics remain largely unknown where temporal variability coincides with remote and inaccessible locations

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